There are, or at least were major commercial operators in New Zealand who operated LOP. I don't know if they still are but when I talked to a former senior pilot he was pretty proud they did.
The argument to the POH argument is that it just says "recommended" best economy and best power. It doesn't say you can only operate at "best economy" or "best power", because often you can't, takeoff for example. So if you can operate richer than those, why not leaner?
I know that some POHs have said said no LOP (or was it separate Lycoming advice?) Anyway, the funny thing is, as I said before if I used the "lean till rough then enrich till smooth" i'm running at least some of the cylinders LOP. Am I operating illegally?
I bet if I offered most of you a homeopathic remedy you'd point out that the data doesn't support its effectiveness. Well, likewise the operation of an engine at 50 degrees ROP. The data shows it is the worst position. Simple. Manufacturers are like regulators and move at the speed of roading tar, so the pressure from people who have gathered real hard data are in my mind of a similar value to those who pressure for regulatory reform.